In electrophotogaphic machines, a photoconductive surface is charged in a uniform manner and then exposed to light reflected from an original to be copied. That light variably discharges the photoconductive material thus providing a latent electrostatic image of the original on the photoconductive surface. The image is then developed through the application of toner which typically is a black powdery substance electrically attracted to the undischarged areas of the image. After development, the image is transferred to a piece of copy paper or some other substrate and fused thereto.
Magnetic brush developers provide high quality development of latent images. These developers generally comprise a rotating hollow shell made of non-magnetic material with numerous stationery magnets disposed within. A developer mix for use in such a system may be composed of a single component or two components. A two-component developer is typically comprised of small steel particles or beads, called carrier, which are magnetically attracted to the surface of the rotating roll by virtue of the magnets disposed therein, and the above-mentioned toner particles. The small steel beads are coated with the black powdery toner, which adheres triboelectrically to them, and carry the toner along the surface of the magnetic brush roll into the development zone where the toner may be electrically attracted from the steel beads to the latent image. A single component developer mix for use in a magentic brush developer will comprise electrically conductive, magnetic marking particles.
Typical magnetic brush developers are shown in U.S. Pat. Nos. 3,999,514 and 4,161,923 to Abbott et al and assigned to the same assignee. The developers described in the aforesaid patents have single developer roll development, and in order to improve the development characteristics of such systems, a number of multiroll development systems have been proposed in which two or more magnetic brush rolls are arranged along the photoconductor. U.S. Pat. No. 4,439,034 to Daniels, which is also assigned to the same assignee, is illustrative of a multiroll system in which developer material is carried from one developer roll to the next in contact with the imaging surface.
As will be noted by reference to the aforesaid patents, the magnetic brush developer rolls include pick-up and transport magnets for conveying the developer material to the development zone. At the development zone a development magnet is located opposite the nip between the magnetic brush roll and the photoreceptor surface. In magnetic brush developers it is important to achieve steady and uniform carrier bead flow from the pick-up area to the development zone. In that manner an ample supply of toner will be provided to develop the latent image. Typically, in order to control the flow of developer material to the development zone, a so called doctor rod or doctor blade is arranged to control the height of developer material on the brush roll, as for example, in aforesaid U.S. Pat. Nos. 3,999,514 and 4,161,923.
Other examples of magnetic brush developer systems utilizing doctor rods are as follows:
IBM Technical Disclosure Bulletin, September 1972, Page 1251, shows a developer with a doctor rod positioned at a transitional area just after the transport magnet.
IBM Technical Disclosure Bulletin, February 1972, Page 2787, also shows a doctor rod positioned just after a transport magnet.
U.S. Pat. No. 4,377,334 to Nishikawa relates to a developer roll with a doctor rod positioned between the development magnet and a transport magnet. The doctor blade is described as providing a given thickness of developer material as it enters the development zone.
U.S. Pat. No. 4,354,454 to Nishikawa relates to a counterflow developer with a doctor rod located similarly to that of U.S. Pat. No. 4,377,334 above.
U.S. Pat. No. 4,347,299 to Ozawa et al relates to a method of controlling toner concentration In the course of the description, a doctor blade is shown positioned prior to a development nip in the conventional manner.
U.S. Pat. No. 4,334,772 to Suzuki relates to a developer with a very small nip gap. A doctor blade is positioned prior to the nip gap in a conventional manner.
U.S. Pat. No. 4,257,348 to Prohaska relates to a magnetic brush developer where the doctor rod is essentially buried in the developer mix. This doctor blade is designed to limit the thickness of the brush.
U.S. Pat. No. 4,200,665 to Suzuki et al relates to toner concentration but shows a doctor rod positioned in the conventional manner although it is nearly buried in the mix.
U.S. Pat. No. 4,030,447 to Takahashi et al relates to a doctor blade which is positioned in such a manner that any material scraped away by the blade is buried in the mix.
In the prior art magnetic brush systems employing doctor rods, as exemplified by the systems described above, in which the doctor blade adjusts the height of the developer brush to a predetermined height, it has been necessary that the doctor rod be set in dependence upon the size of the nip gap between the developer roll and the photoreceptor. This is because the pressure in the nip gap is found to vary according to the setting of the doctor rod. This has meant in the past that doctor rods have in practice needed to be adjustable to enable the setting of the nip gap and doctor rod to be closely matched.
The inventor herein has discovered that this requirement can be avoided by providing a relatively weak pre-nip magnet just ahead of the development magnet, and having the same polarity as the development magnets which creates a tangential field between itself and the transport magnet. Also provided is a mechanism, similar to a doctor rod but more appropriately referred to as a mix barrier, which is arranged in the tangential field area between the prenip magnet and the transport magnet. Preferably, the mix barrier is placed at a transition point fairly close to the transport magnet and not directly in the middle of the tangential field. The result is that developer mix is held to the surface of the magnetic brush by the tangential field in a fairly solid rod-like manner and any additional material is easily peeled away by the mix barrier. The pre-nip magnet serve to pre-condition the pre-nip packing of the developer. As the setting of the mix barrier changes, the pressure in the nip remains constant since the amount of mix going to the nip is determined more by the amount packed into the tangential field than the extra amount peeled away by the mix barrier. If the nip gap changes, the pressure in the nip changes; however, it still retains its independence relative to the mix barrier setting.
Thus, with this arrangement, there is provided a developer configuration which provides relatively uniform pressure in the nip gap regardless of the size of the gap. It also provides independence in the setting of a doctor rod relative to the size of the nip gap. That is, this configuration has removed the criticality of setting the nip-gap and as well as the criticality of setting the doctor rod. By stabilizing flow rates in this way, enhanced and uniform copy quality can be obtained from one machine to another and manufacturability with a minimum of adjustments and calibrations can be achieved.
The benefits, particularly of the preferred embodiments, of the invention include increased design flexibility, increased latitude in mechanical tolerancing, ability to transport a variety of developer mixes, enhanced and uniform copy quality through flow stability and simplicity. Cost reduction is possible since close tolerancing and parts for adjustable doctor rods can be eliminated and the mix barrier can be part of the developer housing. Further cost reduction can be obtained by elimination or reduction of developer and copier tests and calibrations normally required prior to shipment. Cost reduction can also be achieved during field replacements of developers and photoconductor members.